NMR imaging of solids is being explored as a new method of non-destructive analysis for materials in part because of its capabilities for imaging spatial properties of processed polymers and ceramics. Even though imaging of liquids are well known and widely used, the approaches for liquids do not provide satisfactory images of proton rich rigid solids because of the magnetic dipolar couplings and chemical shifts that broaden the resonances. The resulting broad lines reduce the signal to noise ratio of the image and create excitation and detection bandwidth problems. Because the time necessary to acquire a two-dimensional image is proportional to the natural linewidth of the resonance, and since the ratio of the solid to liquid linewidths is typically greater than 1000, artificially narrowing the resonance of the solid has been suggested for facilitating the recording of high resolution images. A number of line-narrowing techniques such as magic angle spinning and multiple pulse sequences have been applied to NMR imaging. Because of the strong homonuclear dipolar couplings, multiple pulse techniques have shown the most promise for imaging .sup.1 H and .sup.19 F nuclei in rigid solids.
A general characteristic of multiple pulse homonuclear dipolar decoupling sequences is the deterioration of line narrowing as the resonance offset increases. This is a fundamental limitation to using this technique in imaging solids because the range of usable resonance offsets restricts the excitation bandwidth in the imaging process. If the pixel width is sensibly set to the residual linewidth, the restricted offset limits the number of pixels across the field of view. Even when the lines are further narrowed for squeezing more pixels into the usable bandwidth, the total number of pixels may still be insufficient for strongly coupled samples because the strength of the unaveraged dipolar couplings determines both the useable bandwidth and the residual linewidth.
The general method for NMR imaging is well-known. A sample is polarized by a magnet, the nuclear spins are excited and their spatial positions marked by phase or frequency encoding by an applied magnetic field gradient. The NMR signal is then detected and the image is reconstructed. Depending on the details of the application, various of these steps are repeated. But the main, perhaps central, feature of NMR imaging is the use of these magnetic field gradients.
Conventionally, magnetic field gradients have been applied as either time independent fields or as fields which oscillate in time during the data collection in NMR imaging. The most common approach has been to turn a gradient field on, allow the gradient field to stabilize and to subsequently carry out an imaging experiment in a stationary field. The imaging experiment would then typically consist of a multiple pulse line-narrowing sequence such as an MREV-8 sequence, for reducing the line broadening from the homonuclear dipolar coupling of the observed nuclei. In such a method, the resonance shifts due to the gradient are inseparable from chemical shifts which arise from chemically distinct nuclear spins. Thus the presence of a static magnetic field gradient not only decreases the resolution of the resultant image, but also the gradient and chemical shifts cannot be distinguished.
Such a decrease in line-narrowing efficiency of a multiple pulse line-narrowing cycle may originate from the radio frequency (RF) pulses being off resonance, or from phase evolution of the nuclear spins during the intervals between the RF pulses. Earlier, imaging with oscillating gradients had been considered for preventing deterioration arising from the RF pulses being off resonance by minimizing the time during which the gradient and RF fields are simultaneously applied. By applying the RF pulses at the zero crossings of the oscillating gradients, the off resonance effects from the gradient during the RF pulses are reduced. Additionally, it is possible to design line-narrowing sequences which eliminate evolution due to the time independent chemical shift and resonance offset while at the same time retaining evolution due to the gradient since the oscillating gradient amplitude changes periodically. However, the oscillating gradient technique does not prevent the deterioration of line narrowing efficiency with increased gradient strength due to spin evolution between RF pulses. Furthermore, the oscillating gradients are restricted to pulse sequences which have a periodic structure within a pulse cycle that is complementary to the gradient period.